At least 30 million American workers are exposed to excessive noise daily with more than one third sustaining noise-induced hearing loss. The cost for permanent hearing loss adds up to billions of dollars per year. Despite the relevance of noise to human health and a long history of noise research, we do not know how excessive noise destroys cells in the organ of Corti (OC). Based on animal studies, several conflicting theories on mechanism(s) of noise damage have been developed but none has been proven unequivocally. It is not known why noise exposure causes different patterns of cell loss in the apical and basal halves of the OC. The lack of knowledge about mechanisms of noise-induced hair-cell death has hindered identification of noise-protective agents. We recently discovered that some dying outer hair cells (OHCs) in noise-exposed cochleae have morphological appearances that are distinct from apoptosis, oncosis and autophagy. If this newly discovered pathway dominates OHC death following moderate noise exposures, this information will revolutionize thinking about the mechanism(s) of noise damage. Some of our proposed studies will determine what death pathways cochlear hair cells follow after chinchillas are exposedto typical workplace noise [i.e., 4-kHz octave band of noise (OBN) or 0.5-kHz OBN at 92-95 dB SPLJ. With Specific Aim #1, the apex-to-base patterns and types of cell death and their timing will be determined in hair cells at 0, 7, 14 and 30 days post-exposure. Post-exposure changes in auditory function will be determined using auditory brainstem response, compound action potential and cochlear microphonic thresholds and distortion product otoacoustic emission levels. The vital dye, trypan blue, will be instilled into one of the cochlear fluid spaces in-vivo, to identify cells with with disrupted plasma membranes. With Specific Aim #2, we will determine the site of action for free-radial damage to OHC plasma membranes compared to the damage produced by noise. Fixed cochleae will be embedded in plastic and dissected as flat preparations. Differential counts of missing and dying hair cells will be made and cells with trypan-blue-stained nuclei will be counted through- out the cochlea. We will compare changes in auditory function to losses of hair cells and other types of structural damage. Our results will provide information on mechanisms of damage from moderate noise so that drugs that will prevent or minimize noise-induced hair-cell death can be developed.